The present invention relates to methods and apparatus for chilling a product and more particularly to methods and apparatus for chilling such product in a blender device with the use of carbon dioxide as a refrigeration medium.
In the course of processing products such as ground meats it is frequently necessary to chill such meat to enable further processing operations to be effected. Large batches of meat products such as ground beef, ground pork, etc. are typically chilled from temperatures of about 40.degree. F. to approximately 28.degree. F. thereby enabling formation of the product in particular shapes such as hamburgers, etc. In order to chill such products, it has been common to utilize blending devices wherein the product to be chilled is mixed with a refrigerant to reduce the average product temperature to a desired level. In the past, blender devices have typically included one or more trough sections in which a mixing element such as a rotatable auger or ribbon screw member is operated so as to pass the products being chilled into contact with the refrigerant thereby reducing product temperature.
Carbon dioxide has been utilized as a refrigerating medium for chilling products in a blending device. Typically, the blender is provided with a cover across the top thereof and means for injecting carbon dioxide through the cover into the interior of the enclosed blender. Those skilled in the art will appreciate that devices known as "snow horns" may be mounted on such a cover and extend therethrough so as to discharge a mixture of solid and gaseous carbon dioxide into the interior of the blender. The cover is typically provided with an exhaust plenum or other means for removing carbon dioxide vapor which is introduced into the blender with solid CO.sub.2 and which forms in the blender upon sublimation of the solid carbon dioxide. In operation, the top of the blender is raised and a batch of product to be chilled is introduced into the blender device. The top is then lowered and carbon dioxide is supplied by snow horns as CO.sub.2 gas and snow into the blender while the mixing elements are actuated. A suitable exhaust fan in the exhaust plenum system is energized so as to remove CO.sub.2 vapor from beneath the blender cover and thereby avoid the buildup of excessive quantities of CO.sub.2 gas in the blender. Upon the product reaching a predetermined reduced temperature, the supply of carbon dioxide is terminated and the product is removed from the blender. This may be achieved by terminating operation of the mixing elements and opening the blender top to permit the product to be `dumped` or opening bottom trap doors. Alternatively, the mixing elements, e.g. screw devices may continue to operate to discharge chilled product through end doors of a blending device of this type. Discharge of the chilled product will enable cleaning of the mixing members and the blender interior. One system for introducing carbon dioxide into a blender through top mounted snow horns is illustrated in U.S. Pat. No. 4,314,451, which is assigned to the assignee of the present invention.
Although the blender devices described above are effective to reduce the temperature of a product, there are several disadvantages attendant to such blending systems. First, it has been found difficult and time consuming to load and unload product into and from a blender as it is necessary to open and close the top cover of the blender. The cover is a relatively heavy member which usually requires hydraulic or pneumatic raising and lowering devices to enable opening and closing thereof. These hydraulic or pneumatic lifting devices are costly and require maintenance to assure effective operation. In addition, the presence of a cover, even in an open position, reduces access to the blender interior and thus renders cleaning of the blender more difficult and time consuming.
The introduction of carbon dioxide through a blender cover as described above is not particularly efficient from a refrigeration viewpoint. It has been found that by expanding liquid carbon dioxide through a nozzle in a snow horn, considerable refrigeration exists in the CO.sub.2 gas thus formed. Although this gas will contact the top surface of product being mixed with solid carbon dioxide, relatively little refrigeration available in such CO.sub.2 gas is utilized in chilling the product. In addition, solid carbon dioxide which is mixed with the product tends to sublime rapidly and a portion of the solid carbon dioxide introduced into the blender will sublime before being thoroughly mixed with the product. Consequently, a significant portion of the refrigeration available from solid CO.sub.2 is not realized in actually reducing temperature of the product being chilled. Thus, blender systems utilizing top mounted snow horns having not been as efficient as desired in terms of utilizing refrigeration available from carbon dioxide.
It has been proposed to introduce carbon dioxide into the lower portions of chilling systems to enable passage of products to be chilled through the carbon dioxide and obtain a greater degree of refrigeration available from the carbon dioxide so introduced. One such system is illustrated in U.S. Pat. No. 3,468,135 wherein a fluidized bed of carbon dioxide is maintained in the lower portion of a chilling device and products are passed through this fluidized bed of CO.sub.2. In this system a set of impellers is utilized to impart continuous motion to the bed of carbon dioxide to fluidize the same and cause solid carbon dioxide particles to contact the product being passed therethrough. A cover is required to retain the solid CO.sub.2 within the chilling system and although impellers may be effective to fluidize carbon dioxide, such devices tend to impart heat into the portion of the system containing the carbon dioxide refrigerant which in turn is effective to reduce the refrigeration of such carbon dioxide. Consequently, the chilling system described in this reference is not considered to be a particularly effective technique for chilling products and is not considered suitable for chilling products such as ground meat.
Accordingly, there is a clear need for a blending system and method for chilling products which does not require a top cover and associated lifting devices yet which enables efficient use of an expendable refrigerant.